This invention relates to electrically propelled traction vehicles and, more particularly, to cooling systems for high power semiconductors used in such traction vehicles.
Traction vehicles such as, for example, locomotives, employ electric traction motors for driving wheels of the vehicles. In some of these vehicles, the motors are alternating current (AC) motors whose speed and power are controlled by varying the frequency and current of AC electric power supplied to the motors. Commonly, the electric power is supplied at some point in the vehicle system as direct current power and is thereafter inverted to AC power of controlled frequency and amplitude. The electric power may be derived from an on-board alternator driven by an internal combustion engine or may be obtained from a wayside power source such as a third rail or overhead catenary.
Typically, the power is inverted in a solid-state inverter incorporating a plurality of semiconductor devices such as diodes and gate turn-off thyristors (GTO). In a locomotive, large off-highway vehicle or transit application, the traction motors may develop 1000 horsepower per motor thus requiring very high power handling capability by the associated inverter. This, in turn, requires semiconductor switching devices capable of controlling such high power and of dissipating significant heat developed in the semiconductor devices due to internal resistance.
In conventional systems the semiconductor devices are mounted on heat transfer devices such as heat sinks which aid in transferring heat away from the semiconductor devices and thus preventing thermal failure of the devices. For these very high power semiconductors it is desirable to use heat sinks having generally hollow interiors through which cooling air can be forced to remove accumulated heat. Each heat sink is mounted to an air plenum and cooling air is blown through the heat sinks and into the electrical circuit area in which the semiconductors are located. The electrical circuit area may include the various control and timing circuits, including voluminous low power semiconductors, used in controlling switching of the power semiconductors.
In locomotive applications the cooling air is typically derived from blowers drawing air from overhead of the locomotive. The incoming air usually contains contaminants including diesel fumes and dust. A spin filter or inertial filter is used to at least partially clean this cooling air. However, all such contaminants are usually not removed resulting in a buildup of contaminants in the electrical circuit area. Such contaminants impede heat transfer and can also lead to electrical breakdown of insulation gaps in the circuitry. Thus, it is desirable to minimize the buildup of contaminants in such circuitry.
An inverter for large AC motor applications typically includes six high power semiconductor devices, such as GTO's, requiring heat sinks and forced air cooling. Each of these devices are generally press packs which require double side cooling for these high power applications. A common arrangement thus requires twelve heat sinks per inverter. On a six axle locomotive, the inverters alone will include 72 heat sinks requiring cooling air. This number of heat sinks requires a high volume flow of cooling air with and concomitant increase in inertial filter capacity. Thus, it is desirable to provide a method and apparatus for reducing cooling air flow requirements while maintaining adequate cooling of semiconductor devices.